Diethylenetriaminepentaacetic acid-gadolinium (DTPA-Gd) is the only one pharmaceutical for use as an MRI agent for which effectiveness as a diagnostic agent in the brain or spinal regions, has almost been established. However, since it is rapidly excreted in the urine after administration, its half-life period in the blood is extremely short, such as about 14 minutes [Hiroki Yoshikawa et al., Gazoshindan, 6, 959-969 (1986)]. Therefore then, it is difficult to make several diagnoses (blood vessel distribution, blood stream distribution, distribution volume, permeation and the like in a lesion) with a single injection. Furthermore, since it is nonspecifically distributed from the interior of a blood vessel to the interstices of tissue cells, sometimes, no clear contrast can not be obtained due to indistinguishable difference in the concentration between normal tissue and a lesion.
Furthermore, since the imaging time in a nuclear magnetic resonance diagnosis method depends upon the magnetic field used in the MRI spectrometer a longer imaging time is required, for example, in the case of using the widely popular low magnetic field. In such case, the condition of a lesion cannot be precisely understood on using DTPA-Gd which disappears from the blood within a short period of time. Thus, diagnosis with DTPA-Gd is naturally limited depending upon the diagnosing site or the particular type of a diagnosing apparatus.
In order to solve these problems, there has been an increased demand for an MRI agent which can localize in a blood vessel for a constant period of time from immediately after administration, stays therein for a relatively longer period of time and has a medium or long half-life period in blood. As a result, paramagnetic metal complex compounds using as their carriers polymer materials such as HSA [Ogan, M. D. et al., Invest Radiol., 22, 665-671 (1987)], dextran [Brasch, R. C. et al., Radiology, 175, 483-488 (1990)], polylysine (JP-A 64-54028) and the like have been studied and developed as prototype imaging agents. However, since all these carriers are polymer compounds having a molecular weight of tens of thousands or more, retention time in the blood is unnecessarily long as from ten and several hours to several days and there are problems with residence in the body and antigenicity and the like.